The present invention relates generally to controlling electrical signals and more particularly to systems and methods for controlling the shape of an electrical pulse in electroporation systems.
It is known that exposure of cells or other biological molecules to intense electric fields for brief periods of time temporarily destabilizes membranes. This effect has been described as a dielectric breakdown due to an induced transmembrane potential, and has been termed “electroporation”. Among the procedures that use electroporation are the production of monoclonal antibodies, cell-cell fusion, cell-tissue fusion, insertion of membrane proteins, and genetic transformation.
The cells or tissue are exposed to electric fields by administering one or more direct current pulses. These pulses are administered in an electrical treatment that results in a temporary membrane destabilization with minimal cytotoxicity. The intensity of the electrical treatment is typically expressed in terms of the field strength of the applied electric field. This electric field strength is defined as the voltage applied to the electrodes divided by the distance between the electrodes. Electric field strengths used in electroporation typically range from 1000 to 5000 V/cm.
For efficient electroporation, it is necessary to control the shape, e.g. time constant of the electrical pulse. For example, electroporation itself occurs within a narrow range of parameters, such as pulse voltage and pulse duration, which is exhibited by a narrow window between electrocution and little or no electroporation. If a pulse with too long a duration or too high a field strength is used, the cells may be lysed (destroyed). If the duration or field strength of a pulse is too low, electroporation efficiency is lost. As an added difficulty, the optimal voltage and time constant varies with the type of cell. The current emphasis on using electroporation to study cells that are sensitive and difficult to transvect makes the control of electroporation conditions particularly important.
One problem in selecting the electroporation parameters is that the sample itself (cells plus buffer) is a significant factor in the load imposed on an electroporator and can have a wide range of resistance values. To provide the desired shape, a selection of capacitors (used to store charge for delivery to the sample) may be used to determine a time constant. Parallel resistors can also be switched-in to supplement the adjustment precision. Switchable high-voltage power resistors are large and costly, however. Additionally, the precision of adjustment achievable is still rather coarse to be able to reduce the number of such resistors and switching elements.
It is, therefore, desirable to provide systems and methods for controlling the shape of the electrical signal in a more efficient and continuous manner.